Vacuum cleaners typically comprise a downwardly directed dirty-air inlet arranged in the cleaner head or a floor tool, through which dirty air is sucked, by means of a motor-driven fan, into dirt and dust separation apparatus. An agitator, such as a brush bar, may be arranged in the mouth of the dirty air inlet so as to agitate the fibres of a carpet over which the vacuum cleaner is passed. The agitator helps to release dirt and dust trapped in the carpet. The agitator is normally driven by means of a motor via a gear or belt transmission system. The motor may be a dedicated agitator motor or a turbine, or may be the vacuum motor that powers the cleaner.
It is desirable to be able to bring the agitator into and out of operation. In the case that the agitator is driven by a dedicated motor, the motor may simply be switched on and off. If the agitator is driven by the vacuum motor, a clutch mechanism may be provided. When the clutch is engaged, torque generated by the motor is applied to the agitator. When the clutch is disengaged, torque is not applied to the agitator.
A problem which may be encountered with vacuum cleaners having an agitator is that, on occasion, the agitator may become jammed by becoming entangled with objects on the floor surface, for example, or by being pressed hard into the floor. Turbine drives have the advantage of being self-limiting in an overload situation, but impose a loss of air-watts at the suction opening. Where the agitator is driven by a dedicated motor, it is relatively straightforward to sense such overload conditions. For example, the rise in current caused by the increase in load can be detected and the motor electrically switched off. However, where the agitator is driven by the vacuum motor, sensing of such overload conditions is more difficult.
Therefore, it has been proposed to provide means for limiting the torque applied to the jammed agitator by, for example, causing the drive belt to slip. However, over time, the belt deteriorates and so needs replacing periodically. This is a nuisance to the user.
A further problem with conventional torque limiters is that they may respond immediately to an overload condition. Such limiters do not allow for momentary overloads such as may occur when the agitator is momentarily pushed into the pile of the carpet, for example.